The present invention relates to nickel-based alloys having relatively great strength at high temperature and generally referred to (e.g. U.S. Pat. Re No. 28,681 issued to Baldwin on Jan. 13, 1976) as superalloys, and, in particular, to articles with different grain sizes in different portions of the article.
The literature shows that, for nickel-based superalloy systems, grain size has a definite effect on mechanical properties. In general, increasing grain size has the effect of increasing high temperature creep rupture properties. The effect of increasing grain size decreases the total grain boundary area and thereby reduces the propensity of grain boundary failure mchanisms to occur.
Conversely, decreasing grain size enhances such properties as high cycle fatigue due to the increased fracture energy required to propagate cracks by enabling the increased grain boundary area to retard dislocation movement. In addition, such properties as impact strength are enhanced by decreasing grain size.
Grain size control in superalloys is generally considered critical in the manufacture of most turbine hardware. Difficulties, however, are routinely experienced by forging, casting and powder forming processes insofar as grain size and uniformity are concerned. The literature shows that increased carbon additions to nickel-based alloys generally aid grain size control in forged products.
Complex cooling schemes and insulated molds designed to control cooling rates of cast alloys have been employed for grain size control. Powder metallurgical component structures are generally restricted by the initial particle size. A dual grain structure for improved properties is described in U.S. Pat. No. 3,741,821 issued June 26, 1973 to Athey et al., but requires complex equipment, extremely close temperature control, and two separate heat treatments. A mechanical process for providing fine surface grains and coarse internal grains is described in U.S. Pat. No. 3,505,130, issued to Paul on Apr. 7, 1970. It requires special surface cold working and a recrystallization heat treatment. Such cold working is impractical on some alloys because of the susceptibility to cold work cracking and, further, the results of such mechanical working can be lost to relaxation at turbine operating temperatures. Thornburg's U.S. Pat. No. 3,597,286 issued Aug. 3, 1971 uses annealed cold rolled iron-cobalt-vanadium alloy and produces a partially recrystallized grain structure with a balance of room temperature mechanical properties commensurate with magnetic characteristics. Such a recrystallization process is generally difficult with age hardenable nickel base compositions and can also result in relaxation at high operating temperatures.
Boron is known to (up to certain amounts, where incipient melting occurs at operating temperature) improve stress rupture properties (see 4,093,476 issued to Boesch). It also acts as a grain boundary ductilizer (see the aforementioned Re No. 28,681).
U.S. Pat. No. 2,763,584, issued to Badger on Sept. 18, 1956 employs decarbonization and deboronization of exterior surfaces of articles for the purpose improving thermal shock resistance. Heating in a hydrogen atmosphere presents problems and is expensive, and the removal of carbon results in loss of control on grain size.